The Senate majority was always in play for 2020, but there's nothing wrong with making the GOP play more defense.
Based on this news, as well as the Super Tuesday results, I'd sort the Senate races into the following tiers:
Sure-fire Dem pickups: Maine, Arizona, Colorado, North Carolina.
Live underdogs: Montana, Iowa, Georgia x 2, Kansas if Kobach is the nominee, Bama
Lotto tickets: South Carolina, Texas, Kentucky, Alaska
Doug Jones and one of the Georgia seats look a lot nicer with Joe Biden at the top of the bill. Thanks Obama. Haven't quite decided what to do with Montana. I know there have been other Democrat statewide winners there in the recent past, and Bullock has won statewide there recently. But a race like this determines is Trump still has coattails.
I put Alaska on there because there's an intra-party fight up there that makes the Alabama Senate donnybrook look downright tame. https://www.adn.com/opinions/editorials/2019/09/28/the-war-for-the-soul-of-the-alaska-republican-party/